Best video format to use for multiple resavings
Don't.


Control the rendering of your codec to a lossless stills format to load into your pipeline. Do so only after having locked your picture edit.
Save to EXR when you must save to a file format, and stay in EXR until your final grade. Even half float EXRs would be satisfactory.
Conform to a display referred format such as JPEG or TIFF at the very end. DPX is the standard here, but is likely a more complex path for reasons that are too vast to list here.
Encode from there.


If you ask a follow up question, I could give you a very standardized production pipe that might illuminate why this is advised.